Oil-in-water-in-oil type (referred to as "O/W/O type" in the following) emulsion composition in which an oil-in-water type (referred to as "O/W type" in the following) emulsion is further emulsified and dispersed in an oil phase, also known as complex emulsion or multiple emulsion, have become important in various industrial uses such as cosmetics, food products, and medicines. Namely, while normal W/O type emulsions have a configuration in which a water phase is simply dispersed in an oil phase, the particle structure of multiple emulsions is such that, as shown in FIG. 1, an inner oil phase 14 is further dispersed in a water phase 12 which has been dispersed in an outer oil phase 10. Accordingly, they are expected to yield special feel of use and effects which cannot be obtained by simple O/W type and W/O type emulsions.
For example, since the inner oil phase 14 is blocked from the external atmosphere by the water phase 12 and the outer oil phase 10, when an oil-soluble material which is easily oxidized is contained in the inner oil phase 14, its stability against oxidization is expected to be improved as compared with the cases where it is contained in the oil phase of O/W type or W/O type emulsions.
Also, in recent years, silicone type bases such as dimethylpolysiloxane have been widely used as a base for external preparations for skin or cosmetic preparations due to their favorable usabilities such as good spreadability, refreshness, and lack of greasiness, as well as their excellent water resistance and oil resistance which make them resistant to being washed out by sweat and water. In particular, in suncare cosmetics, since they are universally used in summer, refreshing feel of use and those not easily washed out by sweat, sebum, or water are required, whereby such silicone bases are being used more frequently.
Of the oil-soluble agents usually added in cosmetic preparations and external preparations for skin, those hard to dissolve in silicone type oils are not a few, however. For example, of the oil-soluble ultraviolet absorbers added in suncare cosmetic preparations, there are many which have a remarkably low solubility to silicone type bases. Accordingly, in cosmetic preparations containing a large amount of a silicone type base, there has been a problem that, when such an ultraviolet absorber is added therein, crystals of the ultraviolet absorber may deposit over time.
In order to prevent crystals from depositing, it has been necessary to restrict the using amount of oil-soluble agents which are hard to dissolve in silicone type oils or concurrently use a large amount of oils such as polar oils to which the agents have a high solubility. Effect of the agent, however, usually depend on its amount added and cannot be sufficiently obtained when the using amount is restricted. Also, when non-silicone type oils such as polar oils are used in a large amount, the amount of silicone type oils is restricted in emulsion compositions, for example, due to problems in terms of formulation, whereby there are cases in which a large amount of silicone type oils cannot be added to make it difficult to yield satisfactory feel of use and resistance to both water and oil.
Further, since silicone type oils are totally different from the other polar and nonpolar oils in terms of structure and property, when non-silicone type oils added in an oil phase of an emulsion such as W/O or O/W together with a silicone type oil, it is difficult to select an emulsifier, emulsification conditions, and the like, thereby making it quite difficult, in general, to obtain an emulsion having a favorable emulsion stability.
These problems are considered to be solved by application of O/W/O type multiple emulsion. Namely, in an O/W/O type multiple emulsion, since an inner oil phase 14 and an outer oil phase 10 are separated from each other by a water phase 12, it is theoretically possible to make two kinds of oil contents such as a silicone type oil and a non-silicone type oil which have different properties coexist independently from each other in one emulsion system. For example, when an oil-soluble material which is hard to dissolve in a silicone type oil and a non-silicone type oil to which the material is soluble are added in one oil phase, whereas the silicone type oil is added in the other oil phase, the material can be prevented from depositing while these components are made to coexist stably in one emulsion system.
As a method of preparing a multiple emulsion, there has conventionally been known a so-called two-step emulsion technique in which an O/W type emulsion formed by using a hydrophilic surfactant 16 is re-emulsified in an outer oil phase in which a lipophilic surfactant 18 has been dissolved. However, the emulsification stability of thus prepared multiple emulsion may be so low that, over time, the inner oil phase 14 and the outer oil phase 10 can merge with each other or the water phase 12 can be incorporated therein, thereby finally dissociating the oils or water. Since the emulsification stability of the multiple emulsion itself is quite low, the above-mentioned effects expected from the O/W/O type multiple emulsion have not been fully attained.
In order to overcome these problems, various attempts have been made. For example, Japanese Examined Patent Publication No. 55-33294 discloses a method in which a milk protein and cane sugar fatty acid ester are used together, while Japanese Examined Patent Publication No. 3-54709 discloses a method in which polyglycerine fatty acid ester is compounded in the outer oil phase. Also, Japanese Unexamined Patent Publication No. 63-30405 discloses a method in which a specific kind of bentonite and dextrin fatty acid ester are respectively compounded in the water phase and the outer oil phase.
However, even in the O/W/O type multiple emulsions prepared by the above-mentioned conventional methods, the inner oil phase still tends to merge with the outer oil phase, thereby reducing the remaining amount of the inner oil phase over time. Namely, since the inner oil phase 14 and the outer oil phase 10 face each other with the water phase 12 and hydrophilic groups of the surfactants 16 and 18 therebetween, there is only a very small barrier for preventing these oil phases 10 and 14 from merging with each other. Also, since the inner oil phase 14 has to be present, it is difficult, physically or in techniques of manufacture, for the water phase 12 to have a small particle size. Accordingly, the O/W/O type multiple emulsion has a greater number of causes for unstableness as compared with the conventional O/W type or W/O type emulsions. Therefore, the products prepared by the above-mentioned methods have a shortcomings that the characteristics of the multiple emulsion are not sufficiently represented in their physical properties.
Also, while an O/W/O type multiple emulsion in which an ultraviolet absorber or an antioxidant is compounded in its inner oil phase has recently been reported in Japanese Unexamined Patent Publication No. 7-101844, since the conventional technique is used for preparing such a multiple emulsion, its emulsification stability tends to be so low that the emulsion condition may be collapsed during a long period of storage, whereby the stability of the agent compounded in the inner oil phase against oxidization over time can not be improved.
Thus, in the conventional multiple emulsions, those which are stable over time cannot be easily obtained. Also, for example, even when an oil-soluble agent is contained in the inner oil phase at the time of preparation, the inner oil phase may merge with the outer oil phase over time, thereby reducing the remaining amount of the inner oil phase and finally incorporating the inner oil phase into the outer oil phase. Accordingly, the agent and the oil in the inner oil phase may migrate to the outer oil phase, whereby the effects expected from the O/W/O type emulsion composition cannot be sufficiently obtained.
Also, in order to improve the stability of an emulsion, the above-mentioned conventional methods include a step for rapidly cooling and plasticizing the emulsion subsequent to the heating step during its preparation. In this case, the heating and rapid cooling steps necessitate a special apparatus and, accordingly, it is quite difficult for thermally unstable ingredients such as vitamins to be compounded.